There are numerous ways to improve the non-flammability of polymer materials. The best and cheapest way to obtain non-flammability is by adding suitable additives to the polymers. Generally the loading levels of the additive should be restricted to as low levels as possible in order to keep the price of the final product at a reasonable level and also to avoid their influence on the physical and processing properties of the polymers.
In order to assure the good flame retarding properties of the additives it is very important to take into consideration the polymer structure with respect to burning properties, the causes for its burning properties and finally how the fire retarding properties are evaluated. Since, the effectivities of every specific formulations are very much dependent on the polymer structure and their degradation mechanisms, the subjective tests based on different standard methods for different products are not sufficient but instead parameters that are related to burning characteristic of the materials such as oxygen index (OI), onset degradation temperatures, rate of thermal degradation, activation energies for thermal degradation, char yield, peak heat release rates (HRR), smoke density and the types of emission products that are formed as a result of burning are very essential to measure and to take into consideration. Beside these fire-relating properties, influence of these additives on the mechanical and the processing properties for the specific polymers are also very important.
Almost all polymeric materials are comprised of organic materials. The major shortcoming of polymeric materials is their burning characteristics. The flammability of some polymers is higher than that of wood and natural fibres. The calorific values for some common polymers such as polyethylene, polypropylene, polystyrene, polymethylmethacrylate are 46000-27000 kJ/kg, whereas this value for wood is 19000 kJ/Kg. In addition, smoke and soot formation, droplets and emission of highly toxic products accompany the combustion of some polymer materials. Thus, the wide application of polymer material makes it necessary to develop fire-retarded materials. As indicated above, the cheapest way to obtain flame-retarded polymer materials is to add suitable types of fire-retardant additives instead of developing new polymers. How these additives then function as flame retardants is very much dependent on the mechanisms in which they interact with the polymer and how the basic burning properties are controlled.
As is evident from FIG. 1, heat is generated under fire conditions. This heat results into pyrolysis or thermal degradation of the polymeric materials forming combustible gases. These combustible gases create, in presence of oxygen, flame and smoke. Since combustion is an exothermic process, it generates more heat resulting into more pyrolysis of the materials and thereby supplying more fuel to the fire. This means that as soon as the material starts burning, the flame reactions just accentuates and it is difficult to stop the flaming process.
The above process sequence suggests that reduction of the flammability of materials in general, requires the following measures:    1 Increase in the thermal stability of materials.    2 Increase in the amount of char formation.    3 Decrease of transport of combustible gases from the material, formed as a result of pyrolysis, to the flame.    4 Reduction of the amount of heat generated as a result of fire.    5 Insulation of the material surface in order to reduce transfer of heat from the fire to the material.    6 Generation of inert gases from the materials as a result of combustion.
This suggests that in order to control the flammability of materials under fire situation, it is essential to control both the condensed-phase reactions in the polymer and the gas-phase reactions in the volatile phase formed as a result of material degradation. Condensed-phase reactions in the polymer phase mainly involve changes in the pyrolytic path of the polymer to favourable conditions so that the formation of combustible gases are substantially reduced.
Formation of lower amounts of combustible gases under fire conditions results into less generation of heat and thereby, reduces material flammability.
The general strategies, reported in the literature, to prepare thermally stable materials with inherently low flammability are to introduce the following in the structure:    1 Incorporation of halogen or phosphorus.    2 Increasing the C to H ratio.    3 Increasing the nitrogen content.    4 Incorporation of conjugation either through aromatic or heteroaromatic ring systems.    5 Incorporation of rigid structure such as semi-ladder or ladder polymers.    6 Incorporation of strong interactions between polymer chains.    7 Incorporation of high degree of crystallinity or cross-linking.
As mentioned above, flame-retardancy of the materials can also be improved if the extent of charring for the materials could be increased under fire conditions. This, in turn, will reduce the amount of combustibles formed under fire conditions. Basic reactions occurring in the gas-phase and the condensed-phase and how different factors affect such reactions are very briefly summarized below.
Gas-Phase Reactions
So far the gas-phase reactions are concerned, all polymeric materials undergo pyrolysis forming combustible gases. These gases are capable of forming hydrogen and hydroxyl radicals, which in turn, may react with oxygen as below:H.+O2→OH.+O.  (1)O.+H2→OH+H.  (2)
The main exothermic reaction in the flame comprise of:OH.+CO→CO2+H.  (3)
In order to restrict or stop combustion, it is essential to stop or reduce the extent of these reactions especially reaction 3.
Condensed-Phase Reactions
Such reactions have been shown to include interaction between the FR and the polymer and occur at temperatures lower than the decomposition temperature of the polymers. Condensed-phase reactions generally comprise of dehydration and cross-linking reactions.
Dehydration is used very frequently in flame retardancy. Dehydration may result as a result of chemical reactions of the hydroxyl groups present in the polymer chains. Crosslinking reactions have also been found very useful because it promotes stabilization of polymers and also contribute to the char formation. Crosslinking has also been shown to increase the melt-viscosity of the polymers and thereby lowering the rate of transport of the combustible gases as a result of pyrolysis.
Physical Effects
Physical effects such as “dilution effects”, “heat sink effects” and endothermic transitions in the additives have also been used to obtain flame retardancy of polymers. Dilution effects involve dilutions of the organic part of the structure and dividing it into small-insulated domains. This means that on pyrolysis, larger amounts of heat are required to reach pyrolysis temperature; therefore less combustible gases are formed thereby generating less heat. The latter effect is also referred to as “heat sink effect”. Thus additives having high specific heats and low thermal conductivities exhibit enhanced flame retardancy. Also the endothermic decompositions of additives been used to reduce the flammability of materials.
Another physical effect, which has been used in flame retardancy is through the formation of impermeable skin of glass or char that hinders the passage of the combustible gases from the pyrolysing polymer to the flame front and at the same time act as an insulating layer for the transfer of heat from the flame to the polymer surface. The latter helps to reduce the pyrolysis of polymers and thereby decreases the formation of combustible fuel gases. The only limitation in obtaining fire retardancy by physical effects is that relatively large amounts of additives (50-65%) are required. Addition of such large amounts of additives may have a substantial influence on the mechanical and processing parameters of the polymers.
Flame retardant (FR) additives reported so far in the literature can, in general, be classified under the following categories as shown in FIG. 2.
An ideal FR-additive for a polymer should be easily incorporated into the polymer, be compatible with the polymer, and not alter its mechanical properties. It should be colourless; exhibit good light stability, and have resistance to ageing and hydrolysis. In the selection of flame-retardants, it is also essential to match the decomposition temperature of the FR with the polymer. In general, the effect of FR must begin below the decomposition temperature of the polymer and continue over the whole range of its decomposition cycle.
Function Mechanism of the Commercially Active Flame Retardant Additives
Halogen Based Flame Retardants (FR)
Halogen-based flame retardants function mainly through the gas-phase reactions. Halogen atom reacts with the fuel forming hydrogen halide. The latter is believed to function as flame inhibitor and consumes hydrogen and hydroxyl radicals as below:H+HX═H2+X.  (4)OH.+HX═H2O+X.  (5)
Reaction (4) was found to be twice as fast as (5) and has been shown to be the main inhibiting reaction. The inhibiting effect was shown to be dependent on the extents of reaction (4) and (1). This is because reaction (1) produces two free radicals for each H-atom consumed, whereas reaction (4) produces one halogen radical, which recombines to the relatively stable halogen molecule. The latter results into lower heat generation and thereby renders fire retardancy.
The flame retardant effectivity of halogens has been found to be directly proportional to their atomic weights as below:F:Cl:Br:I=1.0:1.9:4.2:6.7
Because of higher effectivity of bromine compounds than chlorine compounds, they are used at lower concentrations. It has been shown that on a volumetric basis 13% bromine was found to be as effective as 22% chlorine. Iodine and fluorine compounds are not industrially interesting because the former is less stable and very expensive whereas the latter is very stable. For bromine compounds, their effectivity is also dependent on the type of bromine i.e. if the bromine is an aliphatic or an aromatic one. In general, aromatic bromine is much stable and volatile than the aliphatic ones therefore these compounds evaporate before they could decompose and thereby furnish halogen to the flame. Beside radical trap mechanism, flame retardancy is also affected by physical factors such as density and mass of the halogen, its heat capacity and its dilution of the combustible gases in the flame.
In general, halogen based systems are undesirable because it has been shown that aromatic halogenated fire retardants may give super toxic halogenated dibenzodioxines and dibenzofurans on heating.
Antimony Oxide as FR
Antimony oxide itself has no FR activity but in combination with halogenated compounds it functions as an effective FR. The main advantage of adding antimony oxide is to reduce the amount of halogenated FR, which has been found to negatively influence the mechanical properties of the plastics. As a general rule approximately 25% bromine or 40% chlorine in the form of organohalide is required to reduce the flammability of plastics to an acceptable level. It has been shown that good FR properties of plastics could be obtained by adding only 12% of decabromodiphenyloxide in presence of 5% antimony trioxide. In general, FR plastics containing organohalogen compounds require 2-10 wt-% of antimony.
In such FR systems, SbX3 has been found to be the active component. At lower concentrations, oxychloride (SbOX) is the active component that has been shown to decompose in several endothermic stages at different temperatures to SbX3 as below:

SbX3 is released to the gas phase and undergoes a series of reactions with the volatile combustibles generating less heat. Such reactions involve reactions with atomic hydrogen producing HX, SbX, SbX2 and Sb. Sb reacts with atomic oxygen, water and hydroxyl radicals, producing SbOH and SbO and remove them from the flame reactions. The SbO formed also scavenges H-atoms. SbOX, which is a strong Lewis acid, operates in the condensed-phase by facilitating dissociation of C—X bonds, releasing more halogen and forming char. Char formation inhibits further degradation of polymer and also reduces the surface area. Decrease in surface area results into formation of lower amounts of volatile combustibles due to volatilisation.
A fine dispersion of solid SbO and Sb is also produced in the flame, which catalyses hydrogen radical recombination. The latter results in a lower steady-state concentration of hydrogen radicals resulting into enhanced FR effect. Among the antimony compounds, only trioxide has been found to be most effective compared to tetra- and penta-oxide.
In a recent study (Danish Protection Agency 2001), antimony trioxide has been classified to be harmful (Xn) according to EU directive and must be labelled with the risk-phrase “Possible risk of irreversible effects” (R 40) due to possible carcinogenicity. The substance is reported as teratogenic. The effects in ecotoxicological test are primarily on algae ranging from very toxic to harmful. However, toxicity on crustaceans or fish is very low. Due to these hazardous reasons it is strongly desired that antimony trioxide should be substituted by less hazardous chemicals.
Alumina Trihydrate (ATH)
ATH has been used as a flame retardant and smoke suppressant since 1960's and is available in different particle sizes (PS) ranging from 5-25μ where superfine grades have PS between 2.6-4.0μ and ultrafine grades having PS between 1.5-2μ. ATH is also available in several surface modified grades in order to improve their process ability and compatibility. It has been shown that on thermal degradation ATH undergoes an endothermic decomposition at 200° C. Between 205 and 220° C., this decomposition is slow. Above 220° C., the decomposition becomes very rapid and hydroxyl groups of ATH begin to decompose endothermically. The major endothermic peak at 300° C. represents the decomposition of α-trihydrate to α-monohydrate and subsequently to γ-alumina. Heat of dehydroxylation has been found to be 280 cal/g (298 kJ/mol). ATH in dry form has been shown to contain 34.6% chemically bound water by weight.
Flame retardation by ATH has been shown partly to be due to the heat sink effect, as mentioned earlier, and partly due to the dilution of combustible gases by the water formed as a result of dehydroxylation. Alumina formed as a result of thermal degradation of ATH has been shown to form a heat-insulating barrier on the surface.
The only problem with ATH is that they are required at high loading levels in order to obtain equivalent flame retardancy as by other additives e.g. 100 to 225 parts per 100 parts resins (phr). Such high loadings may affect both the mechanical and the processing properties of the polymers.
Magnesium Based FR
Magnesium hydroxide functions both as flame retardant and smoke suppressant and also functions as an excellent acid scavenger. It releases 30-33% water at about 325° C. and about 50% loading by weight is required to obtain necessary FR properties. Heat of decomposition for Mg (OH)2 is 328 cal/g.
Magnesium carbonate alone has also been found effective as smoke suppressant especially for PVC. It releases about 60% by weight water and 25% by weight CO2 at 230° C. and 400° C. Magnesium based FR's are not very effective as FR-additives and are not very useful because they also require very high loading levels.
Phosphorus-Containing FR
Phosphorus-containing FR includes inorganic phosphates, insoluble ammonium phosphate, organophosphates and phosphonates, bromophosphates, phosphine oxides and red phosphorus. These types of FR can be active either in the condensed- or the vapour-phase or both. The mechanism for flame retardancy varies with the type of phosphorus compound used and the polymer type. However, two principal modes of reaction have been suggested for fire retardancy: dehydration and crosslinking.
During burning, Phosphorus FR produces non-volatile acids, which functions as dehydration catalysts. This, in turn dehydrates the polymer matrix forming graphite type char residue. The latter reduces the formation of flammable gases from the surface. These systems require oxygen and a source of phosphorus acids that are not volatile under the burning conditions. These agents display decreasing efficiency as FR as the oxygen content of the polymer decreases. Vapour phase activity of these agents is primarily observed for non-oxygen containing polymers. Due to their low molecular weights, they are volatile and are lost either during high temperature processing or in the early stages of combustion. Additionally, their transition to the gaseous phase can cause smoke from the burning material to contain toxic phosphorus containing compounds. In order to avoid such problems, organophosphorus functionality has been incorporated in the polymer structure.
Cross-linking, on the other hand, promotes char formation by creating a C—C network and resulting into decrease in chain cleavage. Phosphorus compounds are also used in intumescent systems.
Flame inhibition reactions similar to the halogen radical trap theory have been proposed where PO is generally the most significant species. The main reactions can be summarised as below:H3PO4→HPO2+HPO+PO.H.−PO.→HPOH.−HPO→H2+PO.OH.+PO.→HPO+H2OOH.+H2+PO.→HPO+H2O
Phosphorus has been found to be synergistic with halogen compounds. Compounds containing both phosphorus and bromine in the same molecule have been shown to be much more effective than the blends of bromine and phosphorus additives.
Some of the commonly used phosphorus derivatives are red phosphorus, trialkyl phosphates, triaryl phosphates and halogen containing phosphates such as chloro- and bromo phosphates. It has been shown that by using brominated phosphates, good flame retardancy can be obtained without using antimony. Nitrogen containing polymers have been found to be synergistic with phosphorus compounds.
Phosphorus based FR additives have also been shown recently that they are not environment friendly and therefore their use FR has also been strongly questioned.
Beside the use of above mentioned FR-additives combination of additives have also been reported in order to devise the so called intumescing flame-retardant systems
Almost all intumescing systems comprise, in general, of three basic components: 1) an acid source such as ammonium poly-phosphate (APP), 2) a charring agent such as pentaerythrytol (PER) and 3) a nitrogen blowing agent such as melamine.
In intumescing systems a series of chemical and physical processes occur during the pyrolysis and combustion of the materials. The chemical processes are: decomposition of APP to phosphoric acid, esterification (phosphorylation) of the polyol (pentaerythritol) followed by decomposition and regeneration of the phosphoric acid. Decomposition of melamine helps blow the resulting thick char that finally insulates the substrate from the flame and oxygen. The physical processes, which controls the flame retardancy includes diffusion and transport of combustible and incombustible gases through the polymer melt to the flame zone, transfer of the molten polymer and of the flame retarding molecules to the flame, diffusion and permeation through the char barrier.
All these reactions occur in a very short period of time with various reaction rates. These reaction rates determine the properties of the final char and the flaming behaviour of the materials. By properly designing these rates using proper formulations, it is possible to obtain desired fire retardant properties for different polymeric materials.
It is reported in the literature that fire-resisting properties of PP were strongly improved using the commercial additive Hostaflame AP750 from Hoechst in comparison with the model system APP/PER. This was shown to be due to the different thermal properties of the intumescent composition. The shield developed from the PP-AP750 system was found to show lower thermal diffusivity and high heat storage compared to APP/PER and this was proposed to be the reason why the substrate is protected longer and at higher temperatures than APP/PER. AP750 comprise of ammonium polyphosphate with an aromatic ester of tris(2-hydroxyethyl)iso-cyanurate and bound by an epoxy resin.
JP 57165435A discloses a flame-retardant polyolefin composition to be used as a covering material for electric wire and cable which composition comprises a basic polyolefin and a flame-retardant additive. The flame retardant additive includes halogen-containing compounds and inorganic compounds among which antimony trioxide is mentioned. In order to improve the insulation properties a synthetic insulating oil (e.g. a silicone oil) is added to the composition of basic polyolefin and flame retarder additive. Thus no flame retarding effect of the insulating oil nor of the polyolefin is claimed.
JP 4132753A discloses a flame-retardant polymer composition to be used as a covering material for electric wire and cable which composition comprises a basic polymer, art-recognized flame-retardant additives and a silicone oil. The art-recognized flame-retardant additives mentioned are hydroxyl-containing compounds of Mg and/or Al and Zn carbonate or borate (preferably the carbonate) at substantial concentrations. No flame retarding effect of the basic polymer nor of the silicone oil is claimed.
EP 0960907 A1 discloses a flame-retardant thermoplastic composition comprising a basic thermoplastic resin, a halogen-containing flame retardant, a flame-retardant aid, silicone and magnesium hydroxide particles. All the working examples are disclosing the use of a halogen-containing flame retardant in combination with antimony trioxide as the flame-retardant aid both of which components are undesirable for reasons stated above and thus should be avoided.
US 2006/003006 A1 discloses a flame-retardant composition comprising at least one melamine compound, at least one metal borate, and at least one alkaline-earth metal hydroxide intended for use in powder coating compositions which also include a thermosetting epoxy resin and a hardener for said resin. The powder coating compositions thus obtained are used for encapsulation of electrical and electronic devices. There is no teaching of the use of this flame-retardant composition for the reduction of the flammability of common polymers such as polyolefins, e.g. polyethylene and polypropylene and others.